This invention relates to adaptive signal processing and, more particularly, to adaptive noise cancelling apparatus. The invention has application in systems where it is desired to reduce interference from noise sources that are spatially separate from a target source, e.g., in hearing aids, automatic speech recognition systems, telephony and microphone systems.
Adaptive signal processing systems are characterized by the capability to adjust their response in the face of changing, or time-variant, inputs. These systems are well suited to perform filtering tasks based on automatic "training" in which they continuously monitor their own previously-generated output signals to replace or remove specified components in presently-received input signals. While adaptive systems have broad applicability in areas such as prediction, modeling and equalization, of particular interest here is their application in interference cancelling, i.e., the removal of unwanted noise from input signals.
The prior art offers a variety of noise cancelling circuits. Among these are adaptive beamforming systems, which use spaced arrays of sensors, e.g., microphones, to reduce interference. A simple system, known as the Howells-Appelbaum sidelobe cancler, for example, employs two omnidirectional sensors for receiving input signals generated by target and interference sources. The system filters one of the input signals, the "reference," through an adaptive element and subtracts it from the other, the "primary." The output signal resulting from this subtraction is fed back to the adaptive element which adjusts the filter to minimize the difference between the filtered reference and primary signals. As the filter converges, the signal-to-noise ratio of the output improves--at least when interference dominates the input. See, for example, Widrow et al, Adaptive Signal Processing, Prentice Hall (1985), at pp. 302, et seq.
More complex beamforming systems proposed by Frost, and by Griffiths and Jim, among others, provide improved output signal-to-noise ratios under conditions where the input noise component is not dominant. See, Widrow et al, supra, and Griffiths, et al, "An Alternative Approach to Linearly Constrained Adaptive Beamforming," IEEE Transactions on Antennas and Propagation, vol. AP-30 (January 1982), at pp. 27, et seq.
Unfortunately, even these systems lose their effectiveness when the input becomes dominated by the target itself, or when a target-free sample of noise is not available. Here, the prior art adaptive systems degrade the target signal, producing an output with a lower signal-to-noise ratio than the input. This deficiency becomes of real concern where such beamforming circuits are incorporated into hearing aids and other applications where a target-free reference signal is unavailable and the system must operate at high, as well as low, signal-to-noise ratios.
In view of the foregoing, an object of this invention is to provide an improved adaptive beamforming system.
More particularly, an object of this invention is to provide an adaptive beamforming system which operates effectively over all ranges of input signal-to-noise ratios.
A further object of this invention is to provide an improved hearing aid which processes incoming signals using adaptive beamforming techniques and which continues to operate effectively even when there is relatively little interference in the input signals.